Insecticides are the most important preventative tool against malaria, dengue and filariasis. Even for yellow fever, where a vaccine exists, insecticides are needed to control common outbreaks.
Characteristics that make good public health insecticides (PHIs) are not necessarily shared by modern agricultural or industrial insecticides. For example, DDT (Dichloro-Diphenyl-Trichloroethane) was dropped as an agrochemical in the 1970s because it biodegrades slowly and accumulates in biological systems when widely used in agriculture.
Yet its persistence makes it a powerful PHI. When sprayed inside houses, DDT protects all inhabitants from malaria for approximately one year - no alternative lasts as long at the same cost. Even in places where mosquitoes are resistant to DDT's toxic actions, it deters mosquitoes from entering houses sprayed with the chemical.
Insecticide-treated nets (ITNs) primarily protect only the people sleeping under them and are less effective than generally assumed because ITN distribution does not equate with consistent use. And while the mass distribution of ITNs has undoubtedly saved thousands of lives, mosquitoes are becoming increasingly resistant to the insecticides used on them, partly because the same chemicals are used in farming. Recent studies have identified growing resistance in Benin and Uganda, where free net distribution has occurred for a decade.
We urgently need new insecticides that combine repellency and persistence with strong binding properties (for use with ITNs).
Yet, while government donors and organisations such as the Gates Foundation and the World Bank spend billions on research into new vaccines and drugs, they invest very little in the search for new PHIs. The newest compound recommended by the WHO for malaria control, etofenprox, was made available in 1986.
The private sector has developed every major insecticide since 1940 - for agricultural or industrial purposes. Public funds played a part in early stage research, but private markets drove product development. The market for PHIs is about 1.3 per cent of the total insecticide market (US$400 million out of US$30 billion). So while most large companies who make insecticides - such as Bayer, and Syngenta - are pleased to sell PHIs, they lack financial incentives to develop products specifically for that sector.
And, according to a Boston Consulting Group report backed by the Gates Foundation, the cost of research and development in the agrochemical industry has risen 500 per cent in the past 20 years.
Opposition from environmentalists makes investing in the search for new PHIs even less attractive.
In 1993, the Pan American Health Organization banned the use of pesticides identified by influential environmentalists and in 1997, a World Health Assembly resolution called on WHO member states to reduce reliance on insecticides for controlling vector-borne diseases.
Environmental groups, led by Pesticide Action Network, regularly champion replacing insecticides with "environmentally sound and socially just alternatives". This often means reducing breeding opportunities for mosquitoes and using larvivorous fish to eat mosquito larvae, which can work - but only under specific circumstances. Exclusively relying on these techniques addresses the risks of man-made chemicals, but gives scant consideration to the far more dangerous disease threats. Yet aware of the implacable environmental community, aid groups have been reluctant to defend insecticide spraying.
The WHO has offered no leadership in the struggle to identify and invest in more effective insecticides. Its Pesticide Evaluation Scheme remains underfunded and slow in responding to the needs of the public health community.
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